1. Field of the Invention
The present invention generally relates to surface acoustic wave devices, and more particularly, to a surface acoustic wave device which reflects a surface acoustic wave.
2. Description of the Related Art
In recent years, portable telephones, cordless telephones, and radio communication devices have been rapidly becoming smaller and lighter, and hence widely spread. Those wireless devices include high-frequency circuits.
Such high-frequency circuits include filters. To obtain a small and light high-frequency circuit, a surface acoustic wave device (hereinafter also called SAW device) is used as a filter. The substrate of the SAW device transmits and receives a surface acoustic wave via reflectors, so that desired filter characteristics can be obtained.
The reflectors of the conventional SAW device consist of the grating of electrode fingers disposed on a piezoelectric substrate. The frequency response of the reflector is determined by the reflectivity of each electrode finger. The reflectivity of each electrode finger is determined by its width.
U.S. Pat. No. 5,270,606 discloses a reflector in which the reflectivity of each electrode finger is continuously changed so as to obtain a frequency response having a desired bandwidth. To continuously change the reflectivity, the widths of the electrode fingers are gradually varied.
However, the reflector has hundreds of electrode fingers, and a long period of time is required to design and manufacture the electrode fingers having widths varied from one another.
To eliminate the above problem, several methods for the weighting of finger width have been suggested. One of such methods is a withdrawal method disclosed in U.S. Pat. No. 4,281,301.
FIG. 1 illustrates a conventional reflector.
A reflector 1 has electrode fingers 2 having uniform widths w0 disposed at uniform intervals d0, as shown in FIG. 1. Weighting is carried out by withdrawing the electrode fingers 2 in accordance with predetermined rules. Element 2' denotes spaces where electrode fingers 2 have been withdrawn.
With this conventional reflector, however, as mentioned above, there has been a problem that a long period of time is required to design and manufacture the electrode fingers whose widths are gradually varied so as to continuously change the reflectivity of the reflector.
Another problem with the withdrawal method is that the reflector is weighted by only two levels of reflectivity. This causes the frequency response to differ from the desired characteristics, because accurate weighting cannot be performed.